Tar spot, a damaging fungal disease caused by Phyllachora maydis, has been confirmed in 14 counties across Nebraska, according to updated disease tracking reports. This development marks an unusually early-season appearance, as most of the state's corn crops are currently in the V6 to V9 growth stages, with silking just beginning.

While the presence of tar spot does not yet warrant fungicide application, agricultural experts emphasize the importance of early scouting, particularly in pivot-irrigated fields where dense canopies and prolonged leaf wetness can accelerate disease progression.

Why Early Scouting Matters

Historically confined to the Eastern Corn Belt, tar spot has expanded westward, with Nebraska reporting consistent detections since 2021. The disease thrives in environments with 60-70°F temperatures, high humidity, and at least seven hours of leaf wetness-conditions commonly found under center pivot irrigation systems. This makes fields in those zones particularly vulnerable and in need of regular monitoring.

Fungicide Timing and Yield Impact

According to university-led research from Purdue and Nebraska Extension, fungicides are most effective and economically viable when applied between the R2 (blister) and R3 (milk) growth stages. Even if tar spot lesions are observed earlier, premature spraying may not deliver return on investment. The threshold for action is typically when 5% of the ear leaf area is infected by R3.

Unchecked, tar spot can severely affect photosynthesis, reduce kernel fill, and lead to plant lodging, with yield losses reaching up to 30-60 bushels per acre in severe outbreaks. These reductions can have multi-million-dollar implications across affected regions, impacting crop insurance claims, commodity prices, and input cost planning.

Integrated Management Strategy

To mitigate the spread and severity of tar spot, producers should implement the following best practices:

Conduct frequent scouting, especially in fields with prior disease history or high residue levels.

Use predictive tools like TarSpotter or disease modeling apps to assess canopy risk and weather-driven infection potential.

Practice crop rotation and manage residue to reduce pathogen survival between seasons.

Select corn hybrids with moderate resistance to tar spot where available.

Apply fungicides only when economic thresholds are met, targeting full canopy coverage between VT and R3 stages for optimal effectiveness.

As tar spot continues to emerge earlier each season, timely scouting, informed fungicide strategy, and sustainable management are critical to safeguarding yield potential and minimizing input costs across the Corn Belt.